Our house is on a hill, and has many windows and sliding glass doors. When we first moved here, we had numerous birds fly into the windows or doors and kill themselves.
We even had a cock pheasant fly into the sliding glass door and die. The whole house shook.
I have discovered a simple solution to prevent the birds from flying into our windows.
I bought some window decorations at a dollar store. They stick on to the windows, and are very easily moved or removed. They are not large, but the bird is made aware that there is something solid in front of him and veers away.